[ROCm] Fix CK varlen_fwd binding argument mismatch

[hyoon1]

Summary

Fix an API mismatch between the Python varlen_fwd wrapper and the ROCm CK binding.

Problem

The Python wrapper passes num_splits as the trailing argument to flash_attn_gpu.varlen_fwd, but the CK binding still uses the previous function signature. This causes CK varlen attention calls to fail with an incompatible function arguments error.

Fix

Add the trailing num_splits argument to the CK mha_varlen_fwd declaration and definition.

The argument is intentionally unused. CK continues to initialize num_splits internally and select the split count using the existing heuristic, so this change does not alter kernel behavior.

Testing

Tested tests/test_flash_attn_ck.py on gfx942:

• Built the ROCm CK extension successfully
• Ran the full CK test suite
  • 260680 passed, 152076 skipped

[hyoon1]

@tridao @micmelesse @rocking5566
Could you review this? ROCm FlashAttention CK backend is currently failing on upstream, so would it be possible merge this fix?

[rocking5566]

LGTM, I think we can merge this PR